Heretofore, various kinds of radar systems have been developed for automotive applications and for such purposes as increasing vehicle driving safety. Automotive radar systems so far developed include those that use laser beams or ultrasonic waves, and those that use radio waves, especially millimeter waves in the wavelength region called the millimeter band, such as a frequency-modulated continuous-wave (FM-CW) radar system, an unmodulated continuous-wave (CW) radar system, or a pulse Doppler radar system, as disclosed, for example, in Japanese Unexamined Patent Publication No. 7-209414.
Such automotive radar systems are used to detect a vehicle traveling ahead, an obstacle lying in the path of the radar-equipped vehicle, or a wall or fence on the roadside ahead. Since the scanning range of an automotive radar system covers the road surface, the automotive radar system also receives a signal reflected off the road surface. When the signal reflected by the road surface is received while the vehicle is in motion, the received signal is shifted in frequency from the radar beam signal emitted from the vehicle because of the Doppler effect due to the motion of the vehicle. By detecting this frequency shift, the vehicle speed relative to the road surface can be calculated.
Generally, in a vehicle, vehicle speed is measured by detecting the rotation of the drive shaft and converting its rotational speed into the vehicle speed. However, if slip occurs between a wheel and the ground, the wheel speed does not necessarily agree with the vehicle speed. On the other hand, if provisions are made to measure the relative speed between the vehicle and the ground based on the Doppler shift frequency, accurate vehicle speed can be detected regardless of wheel slip. For example, in the case of a construction machine such as a power shovel, since tires can easily spin, the tire speed does not necessarily agree with the vehicle speed. Also, in a vehicle equipped with an antilock brake system (ABS) which allows the brakes to be applied up to the limit of wheel lock, there is a need to measure the vehicle speed without relying on the wheel speed. Vehicle speed measurement based on road surface detection using a radar system is an effective means for such purposes.
As earlier described, automotive radar can also be used as a road surface detection apparatus for detecting the vehicle speed relative to the road surface. In an automotive radar system, the direction in which the radar beam signal is projected forward of the vehicle may become displaced from the optimum direction for road surface detection because of the vibrations and shocks that the vehicle in motion undergoes.
Japanese Unexamined Patent Publication No. 7-209414 discloses a configuration in which, in an automotive radar system, the center direction, when changing the radar beam projection direction within a plane parallel to the road surface, is made to coincide with the straight forward traveling direction of the vehicle. However, displacements of the radar beam projection direction are not limited to within the plane parallel to the road surface, but should also contain components working in directions vertical to the road surface, that is, upward and downward directions when the road surface is a horizontal surface.
Automotive radar, mounted on the front of a vehicle, is also used widely for measuring the distance to a target such as a vehicle ahead. In such automotive radar, the radar beam axis relative to the upward/downward directions must be maintained horizontal with respect to the road surface so that the vehicle ahead can be detected reliably.
If the radar axis is displaced, there occurs the problem that the detection range of the radar becomes shorter. As a result, in a vehicle equipped with a radar system, diagnosis must be performed to check the radar axis for displacement. However, an apparatus for detecting such an axis displacement in a simple and accurate manner has not been available up to this date.